Retroreflective sheetings and retroreflective articles which reflect incoming light rays toward the light sources are well known, and such sheetings whose retroreflectivity is utilized are widely used in the fields as above-described. Of those, particularly cube-corner retroreflective sheetings and retroreflective articles which utilize the retroreflection principle of cube-corner retroreflective elements such as triangular-pyramidal reflective elements exhibit drastically higher retroreflectivity of light compared with those of conventional micro glass bead retroreflective sheetings or retroreflective articles, and due to the excellent retroreflective performance their utility is yearly increasing.
Whereas, retroreflective sheetings and retroreflective articles using triangular-pyramidal cube-corner retroreflective elements are generally subject to a defect of inferior entrance angularity, because the light rays entering into their reflective lateral faces at the angles less than the critical angle that satisfies the total internal reflection condition determined by the ratio of the refractive index of individual transparent medium constituting retroreflective elements to the refractive index of ambient air, are not totally reflected at interfaces of the reflective lateral faces but are transmitted to the backs of the reflective lateral faces. That is, while they exhibit favorable retroreflectivity within a range at which the entrance angle formed between the retroreflective sheeting surface and incident light is small, there is a problem that the retroreflectivity rapidly drops with increase in the entrance angle.
On the other hand, a triangular-pyramidal retroreflective element can reflect light rays in their entering directions at nearly the whole surfaces thereof, not allowing excessive divergence of reflected light caused by such factors as spherical aberration observed with micro glass bead reflective elements, and hence exhibits excellent retroreflective performance.
Nevertheless, retroreflected light with excessively narrow divergence angle is apt to invite inconvenience in its practical application, e.g., when light rays emitted from head lamps of a car are retroreflected by a traffic sign, the retroreflected light is difficult to reach the eyes of the person driving the car, at positions deviating from the optical axis of the light. Such an inconvenience is particularly enhanced as the distance between the car and the traffic sign is shortened, because of the increase in the observation angle, which is defined as an angle formed between the incident axis of the light and the observation axis connecting the driver and the point of reflection. That is, heretofore known triangular-pyramidal cube-corner retroreflective elements in general are subject to the problem of inferior observation angularity.
Furthermore, because a triangular-pyramidal cube-corner retroreflective element is formed of three reflective lateral faces and its retroreflective performance varies depending on the direction of incident light entering the lateral faces (azimuth angle), the direction of the elements at the time of installation on a triangular-pyramidal cube-corner retroreflective sheeting must be the same. Thus, triangular-pyramidal cube-corner retroreflective elements have a problem of dependency of their retroreflective performance on azimuth angle, i.e., a problem in azimuth angularity.
Heretofore known triangular-pyramidal cube-corner retroreflective elements furthermore have optical axes. An optical axis is defined as an axis passing the apex of a triangular-pyramidal cube-corner retroreflective element, which is equi-distanced from the three reflective lateral faces intersecting each other at substantially right angles and constituting the retroreflective element.
For improving entrance angularity or observation angularity of cube-corner retroreflective sheetings and retroreflective articles, in particular, triangular-pyramidal cube-corner retroreflective sheetings and retroreflective articles, many proposals have been made of old and various improving means have been investigated.
For example, U.S. Pat. No. 2,481,757 to Jungersen describes installation of various forms of retroreflective elements on a thin sheet. Triangular-pyramidal reflective elements which are exemplified in said US patent include those with untilted optical axes, their apices corresponding to the center points of their triangular bases, and those with tilted optical axes, their apices not corresponding to the center points of their triangular bases. The patent states that the sheeting effectively reflects light rays toward an approaching car (improvement in entrance angularity).
As the size of the triangular-pyramidal reflective elements, the same patent states, in terms of depth of the elements, up to one tenth of an inch (2,540 μm). Furthermore, FIG. 15 of this US patent shows a triangular-pyramidal reflective element pair whose optical axes are tilted in positive (+) directions as explained later, the angle of tilt (θ) of each optical axis being presumed to be approximately 6.5°, as calculated from the length ratio between the longer side and the shorter side of the triangular base of the shown triangular-pyramidal reflective element.
Said US patent to Jungersen, however, contains no specific disclosure about extremely small size triangular-pyramidal reflective elements as described later, or no disclosure or suggestion about the desirable size or tilt in optical axis of triangular-pyramidal reflective elements for exhibiting excellent observation angularity or entrance angularity.
U.S. Pat. No. 3,712,706 to Stamm discloses a retroreflective sheeting and a retroreflector in which so called regular triangular-pyramidal cube-corner retroreflective elements whose triangular bases are in the shape of regular triangles are arranged in the closest-packed state with said bases lying on a common plane of a thin sheet. This US patent to Stamm specularly reflects incident light by vapor depositing a metal such as aluminum on reflective surfaces of the reflective elements, to increase the incident angle, whereby improving the problem such as the drop in retroreflective efficiency and the drawback that the incident light entered at an angle less than the total internal reflection condition transmits through interfaces of the elements and does not retroreflect.
However, because the above proposal by Stamm provides a specular layer on reflective sides as a means to improve wide angularity, such drawbacks as that appearance of the formed retroreflective sheeting and retroreflector is apt to become dark, or the metal used for the specular layer such as aluminum or silver is oxidized during use by infiltrated water or air, which leads to occasional decrease in reflective performance. Furthermore, this patent is entirely silent on the means for improving wide angularity by tilting optical axes.
EP 137,736 B1 to Hoopman describes a retroreflective sheeting and retroreflector in which multitude of pairs of tilted triangular-pyramidal cube-corner retroreflective elements having their bases on a common plane are arranged at the highest density on a thin sheet, each pair of said elements having isosceles triangular bases and being rotated 180° from one another. The optical axis of the triangular-pyramidal cube-corner retroreflective element as described in this patent is tilted in negative (−) direction in the sense described in the present specification, the angle of tilt being about 7°-13°.
U.S. Pat. No. 5,138,488 to Szczech also discloses a retroreflective sheet and retroreflective article, in which tilted triangular-pyramidal cube-corner retroreflective elements each having an isosceles triangular base are arranged on a thin sheet in such a manner that their bases are on a common plane at the highest density. In this US patent, optical axes of each two triangular-pyramidal reflective elements, which face each other and form a pair, are tilted toward the common edge therebetween, i.e., in the positive (+) direction as later explained, the angle of tilt being about 2°-5° and the element height being 25 μm-100 μm.
Also in EP 548,280 B1 corresponding to the above patent states that the direction of tilt in the optical axes is such that the distance between the apex of the element and a plane, which contains the common edge of said pair of elements and is perpendicular to the common base plane, is not equal to the distance between said perpendicular plane and the point of intersection of the optical axis with the common plane, the angle of tilt being about 2°-5° and the element height being 25 μm-100 μm.
As above, EP 548,280 B1 to Szczech proposes an angle of tilt of the optical axis within a range of about 2°-5°, inclusive of both positive (+) and negative (−) regions. Examples given in said US patent and EP patent to Szczech, however, disclose only those triangular-pyramidal reflective elements with their optical axes canted with an angle of tilt of (−) 8.2°, (−) 9.2° or (−) 4.3°, having an element height (h) of 87.5 μm.
On the other hand, as a proposal for improving observation angularity, for example, U.S. Pat. No. 4,775,219 to Appeldorn attempts to improve observation angularity with a product in which the V-shaped grooves forming the elements are asymmetric, being slightly deflected from the theoretical V-shaped groove angle forming the cube-corners, and furthermore the deflection causing the asymmetry of adjacent V-shaped grooves is periodically changed.
Such periodical change in adjacent V-shaped groove angles, however, increases difficulty in mold processing. Even if the difficulty could be overcome, number of possible combinations of the deflections is limited and cannot provide uniform spreading of reflected light. Moreover, plural kinds of processing tools for forming the V-shaped grooves, such as diamond-tipped cutting tools, must be used per one V-shaped groove group in single direction. Furthermore, high-precision processing technique is required for forming asymmetrical V-shaped grooves.
U.S. Pat. No. 5,171,624 to Walter discloses triangular-pyramidal retroreflective element with reflective surfaces having a uniform quadratic section, which is formed with a machining tool with curved sectional configuration. Such a triangular-pyramidal retroreflective element formed of the reflective sides having quadratic surfaces allows adequate divergence of light and achieves improvement in observation angularity. Whereas, it is extremely difficult to manufacture such processing tools having curved sections and hence it is very difficult to obtain quadratic surfaces according to an intended design, due to the difficulty in obtaining the tools.
U.S. Pat. No. 5,565,151 to Nilsen attempted to improve observation angularity by removing a part of the reflecting faces, and promoting divergence of retroreflected light with the whereby formed trigonal prism portions and new reflecting faces.
However, the Nilsen invention contains little specific disclosure such as what shape of the trigonal prisms is preferred or at what angles the new-reflecting faces are preferably formed. The invention also requires a special tool for removing a part of the reflecting faces to configure the trigonal prismatic portions. Still in addition, the newly formed trigonal prismatic elements have no retroreflective function but simply serve to spread retroreflected light by dispersing the light into various directions.
In U.S. Pat. No. 4,202,600 to Bruke, et al and U.S. Pat. No. 5,706,132 to Nestegard, et al, attempts to uniformize retroreflectivity of light rays entering at different azimuth angles (azimuthal orientation) are disclosed, by combining plural zones in which element groups having different azimuth angles are arrayed.
As above, those triangular-pyramidal cube-corner retroreflective elements known from U.S. Pat. No. 2,481,757 to Jungersen, U.S. Pat. No. 3,712,706 to Stamm, EP 137,736 B1 to Hoopman, U.S. Pat. No. 5,138,488 and corresponding EP 548,280 B1 to Szczech have the features in common, as illustrated in FIG. 6, that the multitude of triangular-pyramidal reflective elements, which play the kernel role in receiving entering light and reflecting the same, have their bases positioned on a common plane and that each of matched pairs facing with each other have similar configuration and equal height. Such retroreflective sheets and articles constructed of triangular-pyramidal reflective elements with their bases positioned on a same plane are invariably inferior in entrance angularity, i.e., they are subject to a defect that retroreflective performance rapidly drops with increased entrance angle of light rays entering into the triangular-pyramidal reflective elements.
Also those known U.S. Pat. No. 4,775,219 to Appeldorn, U.S. Pat. No. 5,171,624 to Walter, and U.S. Pat. No. 5,565,151 to Nilsen proposed improvement in observation angularity by various means as above-described, but all of the inventions have the shortage that manufacturing of tools therefor or mold processing are difficult.